1. Field of the Invention
The present invention relates to an apparatus for processing substrates with an integrated pumping system for evacuating gas.
2. Description of the Related Art
FIG. 1 is a cross sectional view of a conventional apparatus 15 for processing a substrate 20. The apparatus 15 comprises process chambers 25a, transfer chambers 25b, and load-lock chambers 25c mounted contiguously on a platform 28 with openings for transferring substrates between the chambers. In the process chamber 25a, a process gas is used to etch features, deposit layers of material on a substrate 20, or clean the chamber. The apparatus 15, is isolated in a clean room or semi clean room 30 to separate and protect the substrates from other potentially harmful equipment.
A pumping system 35 is provided to evacuate the gas and create vacuum conditions within the chambers 25a–c. Pumping system 35 typically comprises a high vacuum pump 40, such as a turbo molecular pump; a low vacuum pump 45, such as a rotary blower pump; and a pre-vacuum pump 50a–c, such as a dry vacuum pump. Conventionally, the large low vacuum or pre-vacuum pumps are stored in enclosures or “garages” in a remote location in the fabrication facility. To detect and contain any leaks of the gases being pumped, the air around the pumps is ventilated by a large air collector located at the top of the garage. The high vacuum pump 40 can be housed in the clean room, as shown in FIG. 1, because it is smaller in size, relatively quiet and creates less noise and vibration than low vacuum and pre-vacuum pumps 45, 50a–c. Additionally, the high vacuum pump 40, unlike the low and pre-vacuum pumps 45, 50a–c, exhausts gas to another pump, not to atmosphere. Typically, the inlet 55 of the high vacuum pump 40 is connected to the process chamber 25, and its outlet 60 is connected to a foreline 65a that extends from the chamber to which it is connected to the intake 70 of the low vacuum pump 45, which in turn, is coupled to the intake of the pre-vacuum pump 50a. The pre-vacuum pump 50a exhausts to an exhaust scrubber 72. The pre-vacuum pump 50a reduces the pressure of the process chamber 25a from atmospheric pressure (760 Torr) down to a pressure of about 0.01 Torr; the low vacuum pump 45 reduces the chamber pressure for higher gas flows; and only when the chamber pressure is below 0.1 Torr is the high vacuum pump 40 operated to achieve a high vacuum below 0.1 Torr down to 10−7 Torr. Another type of high vacuum pump is the cryopump, which is used alone or in conjunction with the turbomolecular pump. The pre-vacuum pump 50 is also used in conjunction with a cryopump (not shown) to pump down the process chambers fast. Pre-vacuum pumps 50 and low vacuum pumps 45 are most commonly used in semiconductor processing apparatus. However, some semiconductor processing apparatus also use high vacuum pumps or cryopumps in conjunction with the pre-vacuum and low vacuum pumps to achieve higher vacuum levels within the chambers to which they are connected. A low vacuum pump 45 is essentially a single stage blower typically mounted on top of the prevacuum pump 50 in order to increase the pumping performance of the pre-vacuum pump.
As depicted in FIG. 1 and discussed below, the pre-vacuum pumps and low vacuum pumps have traditionally been placed outside of the clean room in an adjacent room or basement. There are a number of reasons for this remote placement of the pumps. First, the low and pre-vacuum pumps are large pumps that occupy an envelope of about 0.4 m2 each. An “envelope” of space is typically a rectangle having sides defined by the edges of a component or components making up an apparatus. A “footprint” is the envelope of an apparatus with an additional two feet added to each side. As a point of reference, the entire envelope of some processing apparatus like the one shown in FIG. 1, is only about 6 m2. Therefore, six of the low or pre-vacuum pumps could occupy about one-half the space needed for an entire processing apparatus. The space problem associated with the large conventional pumps is magnified by the fact that the pumps are not designed to service more than one chamber and, therefore, each chamber requires its own dedicated pump. The conventional pumps disposed remotely from the processing systems also have intake ports, exhaust ports and other machine interfaces dispersed around various pump surfaces. The distribution of the connection points further increases the space required by each pump.
One problem associated with conventional pumps is contamination and heat generation which necessitate their separation from the processing apparatus. For example, conventional the low and pre-vacuum pumps are mounted in a frame built around their interior components allowing them to be arranged in rows or stacked on shelves in their remote location. With no enclosure to separate the inner workings of the pump from the surrounding environment, any equipment nearby is subject to the discharge of heat and particles from the pump.
Conventional low and pre-vacuum pumps are also heavy, noisy and cause vibration. For example, each pump weighs about 450 lbs. or more and creates noise of at least 65 db. Vibration of a single pump weighing 450 lbs. can exceed 3.0 m/s2. This level of vibration is not allowable near a process apparatus where robot arms are moving delicate wafers to and from process stations and wafer structures are being created in the 0.18–0.25 10−6 m range.
As a result of the location of the pumps relative to the processing system, the forelines 65a–c between the pumps 40, 45, 50a–c and the chambers 25a–c have a large diameter to provide a high conductance pathway that has a reduced pumping load and resistance. The distance between the clean room and the low and pre-vacuum pumps can often require a 50 to 100 foot length of foreline 65a–c. These extended lengths require that the forelines 65a–c have a large diameter to operate the low and pre-vacuum pumps with reasonable efficiency. Typically, the foreline 65a–c is a stainless steel pipe, which resists corrosion from the process gas, having a diameter of 50 to 100 mm (2 to 4 inches). However, the large diameter stainless steel pipe is expensive and a long length of pipe can cost as much as the pump itself. In addition, the large number of elbow joints and connections in the long foreline extending from the clean room to a separate room, have to be carefully sealed with non-corrodible gas seals to avoid leaks and releasing hazardous and toxic gases during operation, which further adds to large capital costs in semiconductor fabrication facilities. Also, the pipes are sometimes heated to reduce the deposition of condensates on the inside surfaces of the pipes, another high expense associated with long, large diameter forelines.
Furthermore, even with large diameter forelines 65a–c, the efficiency of the low and pre-vacuum pumps 45, 50a–c is often decreased by a factor of 2 to 4 because of the loss in pumping efficiency caused by the large length of intervening pipeline. Additionally, the large diameter and long length of the forelines 65a–c provide a large surface area that serves as a heat sink upon which condensates are deposited from the process gas flowing in the lines. In some processes, these condensates are dislodged and loosened by vibrations from the pumps 45, 50a–c and can diffuse back into the chambers 25a–c and contaminate substrates processed therein.
The remote location of the low and pre-vacuum pumps also prevents pressure within the chambers 25a–c from being reduced in a responsive or fast manner because of the distance between the chamber and the pump. Typically, the chamber pressure is measured by the pressure gauge 80 which feeds a pressure signal to a throttle valve controller 90 which opens or closes the throttle valve 75a,b to control the pressure of gas in the chamber 25a–c. However, this system is slow to respond to pressure fluctuations caused by entry of substrates 20 into the chambers 25a–c, transfer of substrates, or changes in a gas flow rate. In addition, the pressure reduction time obtained from “soft start” valves 76 is too slow. The soft gradual pressure reduction is used to prevent moisture condensation when lowering chamber pressure of a load-lock chamber for instance, from atmospheric pressures to the mTorr range, by using two different size valves 76. A smaller valve opening having a low conductance is opened when pumping the chamber down from one atmosphere to about 100 to 300 Torr, and a large sized valve is opened when pumping the chamber down to lower pressures. The two-cycle process provides a soft or gradual reduction in chamber pressure in stages that minimizes moisture condensate in the chambers 25c. However, the time for pressure reduction during the small valve opening step of the process is often excessively long for high throughout fabrication processes, resulting in an increased process cycle time and an inefficient use of the vacuum pumps.
Moisture condensation in the chambers during pump down is a particular problem with long forelines between the chamber and the pump. The problem arises because the volume within the long, large diameter forelines is typically equal to the volume in the chamber. For example, with remotely located pumps, a chamber having a volume of 30 liters may require a foreline also having a volume of 30 liters. When an isolation valve between the chamber and the evacuated foreline is opened, exposing the chamber to the lower pressure foreline, the pressure in the chamber is reduced by as much as one half. The temperature within the chamber can fall to a temperature of less than the dew point causing condensation of water on the walls of the chamber. With long forelines, the problem of condensation in the chamber during pump down is managed by using the soft start valves described above to gradually reduce the chamber pressure and prevent condensation. However, as described above these valves result in high pressure reduction times and represent an additional expense.